1. Field of the Invention
The present invention relates to a liquid crystal display based on an active matrix and a method for producing the same. More particularly, the present invention relates to a liquid crystal display based on an active matrix wherein orientation of the liquid crystal is regulated by a transversely-activated electric field, and a method for producing the same. Furthermore, the present invention relates to a liquid crystal display wherein the display quality is protected against deterioration due to external static electricity, and a method for producing the same.
2. Description of the Related Art
Generally, a conventional active matrix liquid crystal display comprises a nematic liquid crystal sandwiched between a pair of transparent substrates. A voltage is applied between the transparent substrates to align the crystal in an upright position and thereby control the transmission of light (vertical drive mode).
A liquid crystal display based on the above mechanism includes switching elements, an active matrix substrate having various electrodes disposed thereon, a second substrate facing the foregoing substrate, a liquid crystal inserted between the substrates, and polarizing plates arranged on the outside surfaces of the two substrates. Gate and drain electrodes are formed in the horizontal and vertical directions of the active matrix substrate, and switching elements are formed at the intersections thereof.
A transparent pixel electrode is formed in the area surrounded by the gate and drain electrodes, and that area forms a pixel. Numerous pixels having the above constitution are formed on the transparent substrate. A second transparent substrate carries a common electrode thereon. A voltage applied between the electrodes formed on the two substrates changes the orientation of the liquid crystal, to thereby modulate the transmission of light.
FIG. 3(a) is a plan view illustrating the construction of a conventional liquid crystal display based on an active matrix. FIG. 3(b) is a sectional view along the line A-A' of FIG. 3(a) when the display is not activated, and FIG. 3(c) shows the same cross-section when the display is activated.
As shown in these figures, the structure of the liquid crystal display is such that drain electrodes 14 and gate electrodes 10 are disposed vertically and horizontally on transparent substrate 1, respectively, and a transparent pixel electrode 17 is formed in each area surrounded by the two electrodes. The pixel electrode 17 is connected through a switching element 18 and a source electrode 15 to a drain electrode 14.
The switching element 18 often consists of a thin-film transistor. The thin-film transistor includes a drain electrode 14 which contacts a source electrode 15 through a semiconductor layer 30, and has a gate insulating membrane 20 beneath the semiconductor layer 30 and a gate electrode 10 beneath the semiconductor layer 30.
Another transparent substrate or paired substrate (also referred to as a "transparent electrode") 508 facing the foregoing substrate 1 has a common electrode 16 formed thereupon. A liquid crystal composition (as shown by liquid crystal molecules 513) is sandwiched between the two transparent substrates 1 and 508. The crystal orientation of the liquid crystal (the liquid crystal molecules 513) is nearly parallel to the transparent substrates 1 and 508, and is twisted under the influence of the alignment layers OR11 and OR12, from substrate 1 towards the other substrate 508, until the shift in orientation angle reaches 90 degrees.
The operation of this system is described below.
When a specific voltage is applied to the gate electrode 10, the thin-film transistor (or the switching element 18) is turned on, and electric charge moves from the drain electrode 14 through the semiconductor layer 30 and source electrode 15 to the pixel electrode 17. As shown in FIG. 3(c), the liquid crystal (the liquid crystal molecules 513) is aligned in an upright position with a specified orientation under the influence of an electric field developed between the pixel electrode 17 and the common electrode 16, which alters the polarizing activity of the subject liquid crystal. Through this operation, the transmission of light through individual pixels is altered, the light tone of these pixels is modulated, and images are reproduced.
It is known, however, that in such a conventional liquid crystal display, the tone intensity in the display varies depending on the angle from which the viewer watches the display. For example, when viewed from the front with the panel placed upright, or along an axis normal to the panel, one can see images having good contrast. However, when viewing the same image along an axis that is slightly tilted downward, the image appears darker. If the tilted angle is further increased, there will be a boundary beyond which a reverse in tone will occur. In contrast, when viewing the same image along an axis slightly tilted upward, the image appears lighter.
The above phenomenon occurs because a vertically oriented electric field (an electric field having a direction vertical to the transparent substrates) is applied to align the liquid crystal molecules in an upright position. This modulates the polarizing activity of the liquid crystal molecules, such that the direction towards which the liquid crystal molecules are aligned is determined in advance.
To solve the problem of tone variation which depends on the angle from which the display is viewed, a number of solutions have been proposed. Such solutions include, for example, the use of optically compensated films, liquid crystals having a multi-domain orientation, and driving with a transversely oriented electric field.
The method using optically compensated films includes two varieties: one incorporates diffraction and the other is based on the dispersion of converged parallel beams. These methods, however, present problems such as reduced brightness when viewed from the front and tone reversal.
Recently a method has been proposed in which a discotic liquid crystal having a disc-like molecular structure is orderly arranged in accordance with the inclination of nematic liquid crystal molecules and converted into a film. The film is then applied to a polarizing plate. This method solves the above problems.
Multi-domain orientation is a method which divides pixels into a plurality of groups having different orientations, to thereby improve the asymmetric optical performance inherent in a twisted nematic liquid crystal.
The two-domain method or one of the varieties of the multi-domain orientation includes dividing each pixel into an upper part and a lower part, and providing orientation regulating films (alignment layers) each rubbed in a direction opposite the other. As a result, the liquid crystal molecules associated with the two respective parts are oriented opposite each other. This arrangement allows each pixel to provide symmetrical optical performance between the upper and lower halves, and light distribution on the pixel is averaged when viewed from the outside. Accordingly, the problem of tone variation as a function of viewing angle is improved.
Driving by a transversely-oriented electric field has a special significance in this invention. As described above, this method includes controlling the polarizing activity of the liquid crystal with an electric field having a direction transverse to the transparent substrates, as opposed to a conventional liquid crystal display system where the orientation of the liquid crystal is controlled by a vertically-oriented electric field. This method provides a wide-view angle and the display undergoes less change in color tone. Thus, this method is considered to be the most promising means of improving viewability, and many developmental efforts have been directed thereto.
As described in Japanese Unexamined Patent Publication No. 6-160878, driving by a transversely-oriented electric field includes applying an electric field in a direction parallel to the transparent substrates, to thereby twist the orientation of the liquid crystal towards the direction of the electric field and modulate the transmission of light.
FIGS. 4(a)-4(b) and 5(a)-5(b) are schematic diagrams illustrating the working principle of a liquid crystal display based on this driving mode when viewed in terms of a unit pixel thereof. FIG. 4(a) is a plan view of the pixel when a voltage is not applied, and FIG. 4(b) is a sectional view along the line A-A' of FIG. 4(a). FIG. 5(a) is a plan view of the pixel when a voltage is applied, and FIG. 5(b) is a sectional view along the line A-A' of FIG. 5(a).
This liquid crystal display includes a pair of transparent substrates 1 and 508 and liquid crystal in the gap space therebetween, and polarizing plates 505 disposed outside the substrates. On the transparent substrate 1, drain electrodes 14 and gate electrodes 10 are arranged lengthwise and crosswise, respectively. The drain electrode 14 is connected through a switching element 18 to a source electrode 15.
Common electrodes 16 and source electrodes 15 face each other like two combs having teeth directed towards one another. When an electric field is developed between the two electrodes, the orientation of the liquid crystal is altered. A liquid crystal composition (liquid crystal molecules 513) is introduced between the transparent substrate 1 and opposing substrate 508, and is nearly parallel to the transparent substrates 1 and 508 with intervening alignment layers OR11 and OR12 arranged therebetween. The polarizing plate 505 is constructed such that the upper and lower halves have transmission axes intersecting each other at a right angle. This is called a cross-nicol arrangement.
The working principle of this system is described below.
As shown in FIGS. 4(a) and 4(b), when an electric field is not applied, the liquid crystal 513 is oriented with an angle slightly tilted towards the longitudinal axis of the source electrode 15 and common electrode 16 arranged like the teeth of two opposing combs. When a voltage is applied to the gate electrode 10 to turn on the switching element 18, this voltage is applied to the source electrode 15 which generates an electric field E1 between the source electrode 15 and common electrode 16. Then, as shown in FIGS. 5(a) and 5(b), the liquid crystal changes its orientation to align its axis towards the direction of the electric field.
Consider the situation where the liquid crystal (the liquid crystal molecules 513) is anisotropic and has a positive dielectric constant. When the two polarizing plates 505 disposed outside the upper and lower transparent substrates 1 and 508 are arranged so that their light transmission axes have a specified angle AGL1 relative to each other, a voltage applied between the source electrode 15 and the common electrode 16 can alter the transmission of light through the liquid crystal. In more detail, when a voltage is not applied thereto, the birefringence of the liquid crystal is 0 and the intensity of transmitted light is 0. On the other hand, when a voltage is applied thereto, the liquid crystal molecules rotate in the same plane as the direction of the electric field developed between the source electrode 15 and common electrode 16 to exhibit a birefringence property. Then, the intensity of transmitted light is greater than 0. These two states represent gray scales from black to white. Accordingly, in contrast with a system based on vertical application of an electric field, liquid crystal molecules in this system do not assume an upright position when excited. Hence, light distribution on the display does not vary much even when viewed from different angles. This greatly reduces tone variation as function of viewing angle.
However, when a transversely-oriented electric field as described above is used to alter the orientation of the liquid crystal, static charge (static electricity) present on the panel surface (surface of the opposing substrate) generates a vertically-oriented electric field between the static charge and the electrodes. As a result, the liquid crystal tends to assume an upright position, and this interferes with normal viewability of the display.
A conventional system based on a vertically-oriented electric field has paired electrodes formed on transparent substrates and hence is relatively free from disturbance due to static electricity. On the other hand, a system based on a transversely-oriented electric field has working electrodes formed on the active matrix substrate and not on the opposing paired substrate. Accordingly, if static electricity is generated outside the transparent substrates, that is, if static charge accumulates on the surface of the polarizing plates, the static charge tends to disturb the electric field that is concurrently applied to the liquid crystal layer. This, in turn, may result in an uneven display tone.
Furthermore, a liquid crystal display based on a transversely-oriented electric field employs a horizontally placed nematic liquid crystal, and hence is more sensitive to the erratic occurrence of vertically-oriented static electricity than a system based on a vertically-oriented field. Thus, the liquid crystal in a system based on a transversely-oriented electric field readily responds to such vertically-oriented static electricity. This, in turn tends to disturb the display characteristics in terms of uneven tone and reduced contrast.
To suppress the undesirable effects of static charge on a liquid crystal display panel, a method has been proposed in which transparent electroconductive films are disposed outside the transparent substrates sandwiching a liquid crystal. For example, Japanese Unexamined Patent Publication No. 4-51220 discloses a method for producing a liquid crystal display based on a vertically-oriented field in which electroconductive films are applied to the surfaces of transparent substrates opposite those facing the liquid crystal. The electroconductive film can comprise indium tin oxides (ITO) or an ITO coated polyethylenesulfite film.
Furthermore, a method in which an electroconductive film is coated on a polarizing plate is disclosed in Japanese Unexamined Patent Publication No. 1-283504. This method includes applying a hard coat film to the surface of a polarizing plate, and depositing an electroconductive film by metallization on the hard coat film, to thereby produce a polarizing plate having an electroconductive film coated thereon. In this method, low-temperature metallization employing a vacuum metallization apparatus allows for continuous metallization of an ITO compound to produce an ITO-coated polarizing plate.
When an ITO compound or tin oxide (NESA) is used as a material of the electroconductive film, as described in Japanese Unexamined Patent Publication Nos. 4-51220 and 1-283504, sputtering or vacuum deposition is needed to form the electroconductive film. If such a vacuum-operated apparatus is introduced, restrictions must be imposed on the process to prevent damage during film formation, or a process for preparing a hard coat film is added to provide a base so that the overlying metal oxide film has a flat surface.
Furthermore, the above method requires time for cleaning the vacuum chamber which increases the production cost. This is particularly true for the method disclosed in the above Japanese Unexamined Patent Publication No. 1-283504. In this method, the polarizing plate is composed of fibrous transparent unit substrates made of triacetylcellulose which are glued together. Leakage of an agent such as a plasticizer therefrom interferes with the formation of a flat film. This makes it necessary to apply a hard coat film as a base, thus complicating the process.
As described above, if an ITO or NESA compound is applied through metallization by means of a vacuum deposition apparatus to produce an electroconductive film, there is a need to prevent damage cased by this process. As a result, the process becomes complicated and an increase in operating cost is unavoidable.
An alternative method is disclosed in Japanese Unexamined Patent Publication No. 62-67515 in which an electroconductive film formed on a polarizing plate is treated with a cation surfactant. This disclosure, however, lacks concrete descriptions and practicability. Furthermore, because the surface of the electroconductive film which has been treated with a cation surfactant is exposed and unprotected, the subject film is unlikely to be active for a sufficiently long time.
As described above, to prevent the disturbance of a liquid crystal cell due to static electricity, a structure has been adopted wherein transparent electroconductive films are formed outside the transparent substrates which sandwich the polarizing plates and liquid crystal. This method, which allows for production of a transparent electroconductive film having a long-lasting effect and chemical stability, utilizes metallization by sputtering or vacuum deposition of a metal oxide, particularly indium tin oxides (ITO) and tin oxides (NESA).
However, in order to prevent damage during film formation, restrictions must be imposed on the process, or a process for a hard coat must be added which serves as a base for the overlying metal oxide film. Thus, the above methods require additional steps which complicate the process.